An open-flow liquid helium cryostat was constructed for protein crystallography experiments. At temperatures below liquid nitrogen, photoreceptor intermediate states of both reaction center and photoactive yellow protein can be trapped in high yield. In addition, the effects of very low temperature on crystallographic data could be explored using this device. The device consists of a Janis Corp. liquid helium transfer line and a home built nozzle with coaxial warm flow and heating element. The device can run for over 24 hrs using 100 L of liquid Helium at ~50 1 2 K.